In modern commercial aircraft, if not already available electronically, a flight crew makes flight plan entries and modifications through a Flight Management System—Control Display Unit (FMS-CDU). The FMS-CDU is an electronic module containing a keyboard on its lower face half and an electronic display on its upper face half. By keying entries into the keyboard, the flight crew can build or modify a route into the Flight Management Computer (FMC) by typing in a series of waypoints which are then displayed, in text form, on the upper half of the FMS-CDU display.
An additionally provided display is a navigation (map) display. In contrast to the text display of the FMS-CDU, the navigation display graphically depicts the selected waypoints along a desired route. Thus, as the flight crew types entries into the FMS-CDU, these are then displayed graphically on the navigation display.
Current FMCs provide for construction of a variety of flight plans, consisting of point-to-point leg segments and procedural maneuvers. In addition, conventional FMCs provide an autopilot mode where the aircraft automatically flies according to a predefined flight plan by providing lateral navigation (LNAV) and vertical navigation (VNAV) guidance so that the route can be flown. Most commercial airliners can be flown on a constant heading with the autopilot engaged. This allows interception and tracking of a course outbound from a geographical waypoint. However, due to the effect of wind on the airplane's flight path and other factors, the actual heading flown by the aircraft often differs from the predefined flight plan, thus requiring constant adjustment to the airplane heading to maintain the desired course.
In order to facilitate adjustment of the airplane heading to maintain the desired course, many conventional FMCs are also capable of determining the position of the aircraft from navigation systems, such as GPS (Global Positioning System), ILS (Instrument Landing System), IRS (Inertial Reference System), VOR (VHF Omni-directional radio Range) and DME (Distance Measuring Equipment). While these sources can provide adequate positioning information, they each have individual drawbacks. For example, while systems such as GPS systems, which acquire positioning information from satellites, can provide positioning information to an aircraft virtually anywhere, the availability of such satellite-based systems can be limited due to factors such as satellite geometry. And while ILS-type systems provide very accurate positioning information, these types of systems are ground-based systems and are limited to landing procedures at major airports.
Due to the variances in the accuracy of many navigation systems, the United States and international aviation communities have adopted the Required Navigation Performance (RNP) process for defining aircraft performance when operating in en-route, approach, and landing phases of flight. RNP relates to the navigation capability of the aircraft. RNP is a probabilistic approach to evaluating an aircraft's deviation from its intended course, and has been defined by the International Civil Aviation Organization (ICAO) as “a statement of the navigation performance accuracy necessary for operation within a defined airspace.” Currently, several definitions of RNP standards exist, including Boeing RNP, Airbus RNP, RNP-10, and BRNAV/RNP-5. In this regard, according to the Boeing RNP, the navigation performance accuracy can be quantified by a distance in nautical miles, and a probability level of 95% laterally and 99.7% vertically. For example, an aircraft is qualified to operate in an RNP 1 nm lateral, RNP 250 feet vertical airspace if it can demonstrate that the capability and performance of the aircraft's navigation system will result in the aircraft being within 1 nm (nautical mile) lateral of the indicated position on the navigation system at least 95% of the flying time, and within 250 feet vertical of the indicated position at least 99.7% of the flying time.
Expanding upon the lateral navigation accuracy performance standard of 95%, the Boeing RNP defines a lateral integrity containment limit of twice the size of the RNP, centered on the aircraft's predefined path. The integrity containment limit further specifies that the navigation system must ensure the aircraft remains within the integrity containment boundary 99.999% of the flying time.
To determine whether an aircraft is within the RNP or integrity containment limit, FMCs calculate a real-time estimate of the navigation system accuracy, commonly referred to as the Actual Navigation Performance (ANP). ANP represents a measure of uncertainty of position. The ANP is typically calculated by the FMC based upon fault-free performance and integrity statistics provided by the GPS receivers or the aircraft's geometry relative to ground-based navigation aids, and assumptions on the navigation aid survey location error and performance characteristics. The ANP and RNP are then typically displayed on the FMS-CDU in numeric form along with a large amount of other numeric and text information relating to the intended flight path of the airplane. In order to determine whether the ANP is within the RNP, the FMC compares the RNP and ANP values and then sends an annunciation to the display system providing for an “UNABLE RNP” alert when ANP exceeds RNP. This alert does not directly account for RNP changes due to the airplane deviating from the defined path. To account for this, the pilot or other crew member must look at the lateral path deviation displayed on the aircraft Navigation Display and the altitude displayed on the aircraft Primary Flight Display and attempt to determine if the deviation is acceptable for the selected RNP. This display and comparison method of determining whether the ANP is within the RNP requires an unnecessary amount of time, can be very distracting for the pilot and/or air crew member, and is only marginally adequate for low RNP values.
To improve on the ability of a pilot or other crew member to evaluate the RNP and ANP data, prior developments have been made to provide a display depicting navigation performance-based flight path deviation information for use at altitude, also referred to as a Navigation Performance Scale (NPS), an NPS scale, or an ANP-RNP bar. An NPS display refers to a navigation display generated by the FMC for displaying LNAV and VNAV deviations. Such displays are described in U.S. Pat. No. 6,571,155 to Carriker et al., the content of which is hereby incorporated by reference in its entirety. However, NPS scales are only used before final approach procedures. Rather than an NPS display, an ILS or IAN (Integrated Approach Navigation) display is provided upon the final approach segment to a runway during landing procedures. The IAN display is generated by the FMC and supports ILS-like procedures, display features, and autopilot controls for non-precision (non-xLS) approaches. When a precision (xLS) ILS approach is defined and available for a runway, an ILS display is preferred over an IAN display. When ILS is not available, an IAN display is used for non-precision approaches. Unlike NPS displays, IAN displays do not provide deviation scales that depict the relationship between RNP and ANP. The pilot or other crew member must correlate the displayed lateral and vertical path deviations with the numeric RNP and ANP readouts to determine the relationship between RNP and ANP and the lateral and vertical path deviations. This display and comparison method for the final approach segment and landing requires an unnecessary amount of time, can be very distracting for the pilot and/or air crew member, and is inconsistent with flight displays during LNAV/VNAV procedures.